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Ocean acidification – what it means and how to stop it

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The ‘recipe’ for reversing ocean acidification is transitioning to an energy efficient model that relies primarily on renewable sources of energy. Photo: UNDP

In the run up to the Ocean Conference in June, this blog series explores issues related to oceans, seas, marine resources and the implementation of Sustainable Development Goal 14, Life below water.

In the Sustainable Development Goals, the world has set forth a bold new vision for global development and committed to achieving it by the year 2030.   SDG 14 calls for us to “conserve and sustainably use the oceans, seas and marine resources for sustainable development.” While most of the targets in SDG 14 cover ocean issues and challenges that are well known to most, such as pollution and overfishing, one SDG 14 target, 14.3, may not be so familiar:

14.3 Minimize and address the impacts of ocean acidification, including through enhanced scientific cooperation at all levels.

What is ocean acidification, and why is it so important to ocean sustainability and therefore to the SDG agenda?

Chemistry 101

Let’s start with some basic chemistry concepts. Water can be either acidic, basic, or neutral, depending on the relative levels of hydrogen ions it contains. The higher the hydrogen level, the more acidic the solution. This characteristic is quantified in its pH, which runs on a scale from 0-14.

The scale is ‘logarithmic’ meaning that each increment of one is a 10-fold increase or decrease in hydrogen ion concentration. A pH below 7 is acidic, 7 is neutral, and above 7 is basic.

On the whole, the surface ocean clearly falls in the basic range, with pH ranging between 8.0 and 8.3. Marine organisms have evolved in and are therefore finely tuned to the pH of the seawater in which they reside.

What is ocean acidification and how does climate change fit in? 

Due to fossil fuel burning since the industrial revolution, carbon dioxide (CO2) levels in the atmosphere have climbed from about 280 to 400 parts per million. As with the other gases in the earth’s atmosphere, CO2 is largely in ‘equilibrium’ with the surface ocean, meaning a balance is maintained between the amount of CO2 in the oceans vs. that in the atmosphere.

In fact, under this balance, due to the ocean’s high capacity for absorbing CO2, there is about 60 times more CO2 in the ocean than the atmosphere. Under this equilibrium, as CO2 levels in the atmosphere grew rapidly in the 20th century, a sizeable portion – about 30 percent cumulatively – of fossil fuel CO2 dissolved into the surface ocean. 

The positive side of this is that the atmosphere has 30 percent less CO2 than it otherwise would, mitigating to some degree the pace and impact of climate change. But there is a down side as well: upon entering seawater, CO2 immediately reacts with water to form carbonic acid. While considered a ‘weak’ acid (unlike scalding hydrochloric or sulfuric acids), it is nevertheless an acid, which ‘donates’ hydrogen ions to the ocean, lowering seawater pH in the direction of more acidity. 

In the geologic blink of an eye that is the roughly 150 years since the industrial revolution, average surface ocean pH has already fallen about 0.1 unit. Again, because of the logarithmic nature of pH, this seemingly small change represents a 30 percent increase in ocean acidity against pre-industrial times. 

What’s more, in the ‘business-as-usual’ scenario of continued fossil fuel burning, ocean pH is projected to fall an additional 0.3-0.4 units (to 7.6-7.7), the equivalent of a 250 percent increase in ocean acidity. Ocean pH has not changed anywhere close to this much in at least 25 million years and almost certainly never this rapidly in earth history.  

What are the implications for ocean biodiversity and ecosystems?

First, a sizeable fraction of ocean plant and animal life, from tiny but extremely common phytoplankton – the base of the marine food chain - to coral reefs and various shellfish and molluscs, form their shells by fixing calcium and carbonate from seawater into calcium carbonate.  As seawater pH drops, the availability of carbonate ion declines dramatically. Below certain levels, it literally becomes unavailable making it impossible for these organisms to fix their shells/skeletons.

Furthermore, since gases such as CO2 dissolve more readily in colder water, ocean acidification will progress – already is progressing – much more rapidly in the Arctic and Antarctic, where a number of species are already facing challenges in fixing their shells. Under a lower pH ocean future, increasing numbers of calcium carbonate fixing organisms could face dramatic losses or even extinction. This would reverberate throughout the marine food chain as key ‘links’ were diminished or extinguished.

Secondly, ocean acidification also impacts organisms that don’t fix calcium carbonate. Lower seawater pH can weaken a number of organisms’ metabolic processes, from feeding to respiration to reproduction. While it is nearly impossible to predict the precise trajectory of complex ocean ecosystems in these scenarios of increased acidity, there is little doubt that they would be less productive, less diverse and less resilient. In addition, the synergistic impacts of other climate change effects on the ocean, including ocean warming and deoxygenation, will only exacerbate the impacts of acidification.

What can be done?

In 2016, the international community signed the ground-breaking “Paris Agreement” to take aggressive steps to reduce emissions of greenhouse gases that cause climate change. As we have learned above, some 30 percent of CO2 emissions dissolve into the ocean, so every action taken to meet the Paris agreement contributes not only to mitigating climate change but also to slowing and perhaps ultimately reversing ocean acidification.

Every day, we see new signs of progress in this respect, as the costs of renewable energy sources continue to drop and their annual installation levels increasingly exceed that of fossil fuel energy systems – but much remains to be done. In sum, the ‘recipe’ for reversing ocean acidification is the same one as for climate change: transitioning, as quickly as possible, to a low-carbon, energy efficient model that relies primarily on renewable sources of energy to drive our global economy. 

Join the discussion!

We invite you to join the discussion in the recently launched ‘e-dialogue’ on ocean acidification challenges, actions and partnerships at the Ocean Action Hub’s Forum.

Oceans blog series Andrew Hudson Oceans Agenda 2030 Environment Sustainable development Ecosystems and biodiversity Goal 14 Life below water